1. Field of the Invention
The present invention relates to a hydrodynamic bearing and apparatus for driving a polygonal mirror, and more particularly, to a hydrodynamic bearing having an improved air discharging path for discharging air from the bearing when a shaft is inserted into the bearing, and an apparatus for driving a polygonal mirror using the hydrodynamic bearing.
2. Description of the Related Art
A hydrodynamic bearing is widely utilized in an apparatus for driving a polygonal mirror in a laser printer at a fast constant rotation speed. To increase the printing yield, a printing speed of the laser printer must be increased. For this, the polygonal mirror must be rotated by driving apparatus at a high speed for long periods of time.
In order to satisfy this requirement, when the shaft is inserted into the hydrodynamic bearing, an oil film has to be formed around a circumference of the shaft. Since the formation of the oil film is obstructed by air existing in a housing of the hydrodynamic bearing, it is required to discharge the air from the housing when inserting the shaft.
FIG. 1 shows a hydrodynamic bearing having an air discharging structure, disclosed in Japanese Unexamined Patent Publication No. 2001-50257, entitled “dynamic pressure fluid bearing device and motor,” published on Feb. 23, 2003.
Referring to FIG. 1, the hydrodynamic bearing rotatably supports a shaft 1, and includes a housing 11 having a clearance 13 filled with oil 5, and a cap 7 covering one end of the clearance 13 when the shaft 1 is inserted into the housing 11. The housing 11 has a ventilation hole 15 allowing the clearance 13 to communicate with the exterior. When the shaft 1 is inserted into the housing 11, and the oil 5 is introduced into the clearance 13, air in the clearance 13 is discharged outward through the ventilation hole 15, so that the air in the clearance 13 can be eliminated therefrom.
The ventilation hole 15 is widened at an opened end portion 15a thereof by an inclined surface 11a formed in the housing 11. An interface of the oil 5 is maintained at the inclined surface 11a by a surface tension, such that the redundant oil 5 not eliminated from the clearance 13 may stay in the opened end portion 15a. 
The ventilation hole 15 has to be machined in a direction perpendicular to an inserting direction of the shaft 1 into the housing 11. When the hydrodynamic bearing is very small (a few mm in height), it is necessary to machine the ventilation hole having a diameter of several microns. However, such a machining process is very difficult, remarkably increasing a manufacturing cost and reducing productivity.